The invention relates to an electrically contacting fastening device having a threaded bolt and a nut, between which an electric contact element is arranged.
These types of contact connections are known in general, for example, from German Patent Document DE 20 2005 021 114 U1 or German Patent document DE 10 2008 010 352 A1.
In the case of the electrically contacting fastening of the nut and the threaded bolt having a contact element arranged in-between, high tightening torques are required at the nut, in order to maintain the predefined electrical resistance of the contacting for the entire service life. In the case of arc-ignited aluminum fastening and contacting devices having a threaded bolt, a contact element, preferably a cable lug, and a nut, a lubricant is therefore used for screwing the nut to the threaded bolt, because otherwise a plastic deformation of the thread and a type of welding-together of the bread material generally called a “galling of the thread”, may occur when the nut is tightened.
Conventionally, such components used in mass production (for example, within the scope of a motor vehicle) of the fastening device (threaded bolt, nut) in the drum process, are coated with a lubricant, particularly all locations on the nut being coated with the lubricant. Although the lubricant promotes the fastening suitability at high tightening torques for ensuring a secure mechanical connection, it has poor electric conductivity and, because of its sliding characteristics, increases friction corrosion at the contact surfaces with respect to the contact element during its service life.
It is therefore an object of the invention to provide an electrically contacting fastening device having fastening elements made of aluminum, which, for its entire service life, ensures an electric contacting of the fastening element with respect to a contact element. The electric contacting is improved compared to the state of the art and permits an at least equal clamping force of the fastening elements onto the contact element, without the occurrence of material damage to the fastening elements.
This and other objects are achieved by an electrically contacting fastening device having a threaded bolt and a nut, between which an electric contact element is arranged. The threaded bolt has a contacting portion for providing planar contact of the contact element and a threaded portion. The nut can be fastened on the threaded bolt by way of the threaded portion and, on the face side, has a contacting area for the planar contact with the contact element. The threaded portion of the threaded bolt is provided with a lubricant coating, and the contacting portion of the threaded bolt, the contact element and the face-side contacting area of the nut are kept free of lubricant, in order to ensure an optimal contacting for the entire service life. Since the problem of a welding together of material occurs mainly in the case of aluminum or other soft metals as a result of high tightening torques, the invention particularly relates to such fastening elements (threaded bolts, nuts). The permanently lubricant-free faces of the nuts and of the contact portion of the threaded bolt additionally reduce the expenditures for the coating and cleaning respectively, because “permanently lubricant-free” is defined according to the invention such that lubricant is or was applied at no point in time. An embodiment with a lubricant coating and a subsequent cleaning is therefore not “permanently lubricant-free” according to the invention.
In an embodiment, the nut is designed as a cap nut which is placed onto the threaded bolt and is screwed to it, the face of the cap nut forming the contact area remaining lubricant-free. Because the threaded bolt is already coated with lubricant, the thread of the cap nut may also be free of lubricant. An excess of lubricant on both fastening elements may also result in an overloading of the material and a wetting of the contact surfaces, which is undesirable.
In an advantageous embodiment of the invention it is provided that the threaded bolt has a planar and disk-shaped contacting portion. Likewise, it is advantageous that the face-side contacting area of the nut has a planar design in order to achieve a best-possible contacting and sealing of the contact area. The contact element preferably has a coplanarity in a range of 30-65 μm, more preferably 30-50 μm, respectively, with the contacting area of the nut and the contacting portion of the threaded bolt.
Such a coplanarity permits a tight, corrosion-resistant fastening between the threaded bolt, the contact element and the nut for the entire service life.
In a further advantageous embodiment, it is provided that the contact element (for example, the cable lug) has a tin coating at least in the contact area, which tin coating preferably has a thickness of 10-15 μm. The defined thickness ensures a compensation for irregularities but simultaneously prevents chipping or detachments.
As the method of providing such an electrically contacting fastening device, it is provided that the lubricant coating of the threaded bolt is applied by dipping or by a flowing around. In an advantageous embodiment, the threaded portion of the threaded bolt is guided through a lubricant bath, while the threaded bolt is held on the contacting portion. The contacting portion may, for example, be held on a rail device moved in an automated manner, while the threaded portion is guided through the lubricant bath, suspended, as a result of gravitational force.
Optionally, the above characteristics can be combined, if technically conceivable.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.